Cylindrical glass fiber reinforced plastic underground tanks are non-corrosive and generally longlived, but occasionally failures occur due to improper installation. A proper installation requires that there be no voids beneath the tank and that the backfill material be properly compacted. Pea gravel is the preferred backfill material, with sand being the next best choice. Not all contractors follow the recommended installation procedures, and voids still occur under a tank, with the possibility of causing subsequent failure.